Method for fiber coating with particles

ABSTRACT

A system and method for uniformly coating one or more fibers (10) with particles of a material is described. The method uses a vibrator preferably an acoustic speaker (17b) in a housing (17c) to fluidize the particles (P 1 ) in a chamber (17 or 30) to deposit them on spread fibers (10b). The fibers can be in the form of a tow of fibers. After the particles are coated on the fibers, the particles can be bonded to the fiber such as by using a heater (19). The resulting product has a uniform deposit of the particles and in the case of the tow of fibers can serve as a prepreg for laminate structures to be produced from the coated tow of fibers.

This is a division of copending application Ser. No. 07/484,779 filed onFeb. 26, 1990, now abandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a method and system for the productionof coated fibers. The present invention particularly relates to acomposite prepreg with either a thermoplastic or thermoset matrix onfibers in a tow which is suitable for use in primary and secondarystructural preform methods with significant cost, speed andenvironmental advantages. The present invention particularly relates toa method and system which uses a vibrating means in a housing tofluidize particles and to coat them on the fibers.

(2) Prior Art

High-speed, low-cost processing methods are required to produce prepregcomposite materials for high volume applications with either thermosetor thermoplastic matrices dispersed through a tow of fibers.Conventional processes like hot melt prepregging rely on the creation ofa low viscosity matrix phase early in the process which is then forcedaround and through the fiber tows at high pressure. Wetting ofindividual fibers at the microscopic level is the ultimate goal Forthermoset materials, dilution in solvents has been relied upon forcomposite fabrication or if the reaction window is long enough, elevatedtemperatures are used to lower resin viscosity. Thermoplastic matricesare inherently high in viscosity and elevated temperature and pressurecan not always be used. Solution methods are commonly used.Environmental and health concerns as well as the costs associated withusing and reclaiming high boiling solvents makes their use with thesemethods undesirable. Many high performance matrix resins are not solublein any solvents and they are therefore not capable of being used in highvolume applications at the present time.

The key feature to any successful processing method is the ability toplace the polymer around the fibers to form the prepreg. A viableprocessing method should place the polymer in its final position on thefiber surface and should be applicable to any matrix.

(1) Solution Processing

This method is used with both thermoset and thermoplastic matricesPolymer is dissolved in a solvent and the fiber tow is impregnated withthe resulting low viscosity solution (Turton, N., and J. McAinsh, U.S.Pat. No. 3,785,916). Complete removal of solvent after impregnation is astringent requirement and is often a difficult step. Methylene chloride,acetone and N-methyl pyrrolidone are widely used as solvents. Epoxies,polyimides, polysulfone, polyphenyl sulfone and polyether sulfone aresome of the matrices which have been solution-impregnated.

(2) Slurry Processing:

Polymer particles are suspended in a liquid carrier and the fiber tow ispassed through a slurry tank which contains polymer particles. Polymerparticles are trapped within the fiber tow. Taylor (Taylor, G. J., U.S.Pat. No. 4,292,105) outlines a process whereby the particles aresuspended in water which is thickened with a material such aspolyethylene oxide to increase the viscosity to 400-3000 cP at 25° C.O'Connor (O'Connor, J. E., U.S. Pat. No. 4,680,224), points out problemswith this method which include finding the right concentration ofslurry, maintaining the optimum concentration in the resin tank andaccumulation of excess resin at the die entrance (where the impregnatedtow is consolidated into a tape or flat sheet). The minimum voidcontents in the processed tape were about 2 to 4%. Another method(Dyksterhouse, R., Dyksterhouse, J.A., U.S. Pat. Application No.114,362, November, 1987) impregnates the fibers in a gelled impregnationbath with plastic flow characteristics, shear-thinning behavior and apolymer binder in which the polymer particles are uniformly suspended APEEK-carbon fiber void-free composite was made by impregnating G30-500carbon fibers in a gelled impregnation bath having a viscosity of 81000cP.

(3) Melt Impregnation

Direct impregnation of the fiber tow with molten polymer is possible Forthermoset matrices like epoxy, temperature and reaction kinetics allowfor a continuous melt impregnation before reaction For thermoplastics,two approaches have been used: (a) a cross head extruder feeds moltenpolymer into a die through which the rovings pass (Moyer, R. L., U.S.Pat. No. 3,993,726), and, (b) the fibers pass through a molten resinbath fitted with impregnation pins to increase the permeability of thepolymer into the tow. The impregnation pins can be heated to decreaseviscosity locally to further improve the impregnation process (Cogswell,F. N., et al. U.S. Pat. No. 4,559,262). In either case, the forceexerted on the fibers e.g. die pressure for the crosshead extruder areextremely high and can cause fiber damage. The resulting prepreg usuallylacks tack and drape.

(4) Film Stacking

of fiber reinforcement either in the form of unidirectional tows orwoven fabrics are stacked with thermoplastic sheets as the matrixmaterial and consolidated under pressure for long times (Lind, D. J.,and Coffey, V. J., U.K. Patent 1,485,586). This method is widely useddue to the relative ease of manufacture. Disadvantages include highresin content, the uneconomic (labor-intensive) nature of the processand difficulty in impregnating the fiber tow (high pressure forces thefibers together) with the high viscosity matrix material.

(5) Fiber Co-mingling

A thermoplastic matrix can be spun into a fine yarn and co-mingled withthe reinforcing fiber tow to produce a co-mingled hybrid yarn (Clemans,S. R., et al., Materials Engineering, 105, 27-30 (March, 1988)). Thesehybrid yarns can be consolidated to form composite parts. An advantageof this technique is the drapeability of the hybrid yarn. The high costinvolved in producing thermoplastic yarn and weaving it with thereinforcing fibers is a disadvantage.

(6) Dry Powder Impregnation

Dry thermoplastic powder is introduced into a fiber tow which is thenprocessed by heating to sinter the powder particles onto the fibers Thistechnique was first employed by Price (Price, R. V., U.S. Pat. No.3,742,106) who passed glass roving through a bed (either fluidized orloosely packed) of thermoplastic powder. Polypropylene particles with anaverage diameter of 250 microns were used. The particles stick to thefibers due to electrostatic attraction. The tow is then heated andpassed through a die to produce an impregnated tow. The impregnation ismacroscopic, i.e. the particles coat clusters of fibers rather thanindividual fibers leaving unwetted areas and voids. The process istargetted mainly at producing short fiber reinforced thermoplastics.Ganga (Ganga, R. A., "Flexible Composite Material and Process andApparatus for Producing Same", AT0113 (DPI8176); "Procede de Fabricationd'Objets Composites Obtenus", FR 2548084-Al, June, 1983), fluidizedpolyamide particles less than 20 microns in size in a fluidizationchamber, impregnated glass rovings and covered this with an outer sheathof a second material of lower melting point than the impregnatedparticles. The second sheath was extruded onto the tow. Muzzy (Muzzy,J.D., ASME Symposium on Manufacturing Science of Composites, p27-39(April, 1988)) recently demonstrated the ability to manufacture prepregby passing a spread tow through an electrostatic fluidized bed of PEEKpowder (50 microns).

Dry powder processes offer the optimum potential for the creation of ahigh speed, low-cost process if the creation of a void-free compositewith controllable volume fractions can be achieved An ideal processshould have the following characteristics:

(i) It should be independent of matrix viscosity. Most high performancethermoset and thermoplastic matrices are either very reactive at thehigh temperatures necessary to reduce their viscosity or are highlyviscous (10³ to 10⁵ Poise) above their softening point (amorphous) ormelting temperature (semi-crystalline). A viable dry powder processcircumvents this problem by coating fibers individually with therequired amount of matrix so that flow of the polymer takes place onlyover submicron distances between particles rather than centimeterdistances from the outside to the inside of the fiber tows.

(ii) It should avoid the use of binders or solvents which have to beevaporated during the latter stages of the processing cycle This canalways be a source of voids which have a deleterious effect on themechanical properties of the composite.

(iii) The average particle size of the material used is preferablyapproximately the same as the dimensions of the fiber for optimumimpregnation. A significantly larger particle size will cause bridgingand restrict impregnation due to physical limitations which results in anon-uniform distribution of resin between the fibers.

(iv) The concentration of powder particles in the "impregnation chamber"where they meet the fibers should be constant and controllable at alltimes.

(v) The mechanism of adhering the particles to the fibers should becontrollable and independent of environmental conditions.

(vi) The resulting prepreg tape should be in a form that is flexible anddrapeable so that complex parts can be formed easily.

(vii) The process should require minimal energy use, be free of laborintensive steps, capable of operating at high speeds and be capable ofscale-up to large sizes.

Patents which relate to fiber handling which are of general interest areU.S. Pat. Nos. 2,244,203 to Kern; 3,017,309 to Crawford; 3,304,593 toBurklund; 4,534,919 to MicAliley et al; and 4,714,642 to McAliley et al.

OBJECTS

It is therefore an object of the present invention to provide a drypowder process for the uniform coating of fibers with particles of amaterial which meets all of the criteria previously discussed. Further,it is an object of the present invention to provide a process which isrelatively simple and economical. These and other objects will becomeincreasingly apparent by reference to the following description and thedrawings.

IN THE DRAWINGS

FIG. 1 is a schematic, front cross-sectional view of the system of thepresent invention, particularly a spreader 14 for a conveyed tow offibers 10, a particle coating chamber 17, and a heater 19 for bondingthe particles to the fibers 10.

FIG. 1A is a cross-section along line 1A--1A of FIG. 1 showing thecross-section of a chamber section 17d for aerosolyzing the particles P.

FIG. 1B is a cross-section along line 1B--B of coating section 17a ofFIG. 1.

FIG. 1C is a cross-sectional along line 1C--1C of heater 19 of FIG. 1.

FIG. 1D is a cross-section along line 1D--1D of spreader 14 of FIG. 1showing the speaker 14a.

FIG. 1E is a cross-section along line 1E--1E of of FIG. 1 showing thespeaker 17b in housing 17c.

FIG. 2 is a perspective view of an enlarged view of the spreader 14illustrated in FIG. 1, particularly showing the mounting of the rods 14brelative to the speaker 14a.

FIG. 3 is a front cross-sectional view of another type of chamber 30 foraerosolyzing the particles P₁, particularly illustrating the mounting ofthe chamber 30 in a casing 31. FIG. 3A is a cross-section along line3A--3A of FIG. 3.

FIGS. 4A to 4B are photomicrographs of an impregnated tow of fibers(FIG. 4A) and composite formed after heating and consolidation (FIG.4B).

GENERAL DESCRIPTION

The present invention relates to an improved system for coatingparticles of a material on a fiber which comprises: chamber means aroundthe fiber for containing the particles to be deposited on the fiber; andvibrating means mounted in the chamber means such that the particles arefluidized and deposited on the fiber when the vibrator means isactivated.

The present invention also relates to an improved system for coatingparticles of a material on a fiber which is to be conveyed through thesystem which comprises: chamber means around the fiber for containingthe particles to be deposited on the fiber; and vibrating means mountedin the chamber means such that the particles are fluidized and depositedon the fiber as the fiber is conveyed through the system when thevibrator means is activated.

Further, the present invention relates to a system for spreading fibersand coating a powder on the fibers of a fiber tow to be conveyed throughthe system which comprises: feed means for feeding the tow of fibers;spreader means for spreading the tow of fibers from the feed means;chamber means around the tow of fibers for containing the powder to bedeposited on the fibers; an acoustic speaker means mounted in thechamber means such that the powder is fluidized and directed at the towof fibers when the speaker is activated; and take-up means for thecoated tow of fibers.

The present invention also relates to a method for coating particles ona fiber which comprises: providing a system for coating particles of amaterial on a fiber comprising housing means around the fiber forcontaining the particles to be deposited on the fiber, and vibratingmeans mounted in the chamber means such that the particles are fluidizedand deposited on the fiber when the vibrator means is activated;providing the fiber in the chamber means in the system wherein the fiberis coated with the particles when vibrating means is activated andactivating the vibrator means to coat the fiber.

The present invention further relates to a prepreg for a laminateproduct which comprises a tow of fibers having beads of a thermoplasticpolymer along the length of each fiber in the tow of fibers.

The present invention provides a low-cost powder processing method forproducing a composite prepreg suitable for use with any preformprocessing method. Polymer powder particles are preferably acousticallyfluidized and dispersed in a large chamber. Fibers are preferably alsoacoustically separated and then drawn through the chamber into thefluidized powder. Individual fibers are coated with particles uniformlyand the particles adhere to the individual fibers in their finalposition so that heating can fix the particles in place. The resultantpreferred prepreg is a flexible, drapeable material which issubsequently processed to produce a void-free, fully densifiedcomposite.

The process can be scaled-up and controlled with automated techniques.It can be used with any polymeric, metallic or ceramic matrix that canbe produced in a powder form. Prepreg materials have been produced wherethe volume fractions of fiber controlled to two percent total derivationover a wider range of concentrations. The method has been used forfabricating prepreg specimens widths of 10 mm to 50 mm.

This invention particularly relates to a low-cost/high speed compositeprepreg method. The method produces continuous fiber composite prepregwith fiber volume fractions controllable to within one percent. Themethod is continuous, requires no solvents, can be used with anyfiber-matrix combination and is scalable to any prepreg size. Capitalequipment and energy input for the process are low making this a viablelow-cost processing method. The method is based on the use of acousticenergy to overcome gravitational forces as opposed to gas flow inconventional fluidization. Agglomeration and channeling of cohesivepowders (approximate size <20 microns) makes their fluidizationextremely difficult to accomplish by conventional gas fluidizationtechniques. The present invention, however, makes it possible tofluidize and entrain particles of any size at any desired concentration.

An acoustic speaker is preferably used to both separate the fiber towsinto individual fibers and to entrain small polymer matrix particles inair or any suitable gas, whether reactive (e.g. oxygen) or non-reactive(nitrogen), in a chamber through which the fiber passes. The particlesadhere to each fiber filament providing a uniform coating when subjectedto heating so that the prepreg can be processed to any degree ofconsolidation downstream. The resulting prepreg is drapeable and can beused in weaving or preform operations.

The acoustic speakers have preferably a rating of about 8 ohms and 100watts and a sound level of about 80 to 130 dB. The frequency ispreferably between 11.5 to 12 Hz. Essentially any frequency can be used,preferably corresponding to the natural frequency of the coatingchamber, including the end closures and the powder. Preferably thefrequency can be anywhere in the audio to ultrasonic range and ispreferably between 1 to 20,000 Hz which includes the audible range.

SPECIFIC DESCRIPTION

A model system consisting of carbon fibers (7.2 microns diameter) and asmall diameter polyamide powder particles (9.2±4 microns averagediameter and range 5 to 15 micron <20 microns) was used. Acoustic energywas responsible for aerosolization of the particles.

A schematic of the system is shown in FIGS. 1 and 1A to 1E. A fiber tow10 was unwound from a spool 11. The tow 10 was passed through a guidering 12 and between nip rollers 13 before passing over the spreader 14.The spreader 14 has a high sound level (80 to 130 dB) audio speaker 14ato separate the tow 10 into its individual filaments or fibers withoutdamaging the fibers of the tow 10. Nip roller 13 and nip roller 15maintained a constant slack in the spread tow 10a in the region abovethe speaker 14a. Circular cross-sectioned rods 14b maintain theseparation of the fibers in the spread tow 10a. The tow 10a zig-zagsover and under the rods 14b.

FIG. 2 is an enlarged view of the spread tow 10a showing the rods 14bmounted with axis in parallel on spaced apart holders 14c. As can beseen the speaker 14a is mounted adjacent and below the rods 14b.

The width of the spread tow 10a of fibers is controlled by guides 16 andthe spread tow 10a is passed 25 through a coating chamber 17 with acoating section 17a where aerosolized particles P₁ impact and coatindividual fibers of the spread tow of fibers 10a. The particles P₁ areaerosolized by a second speaker 17b mounted in an enclosed housing 17con the bottom of chamber 17. Diaphragm 17e made of rubber, separates thespeaker 17b from the particles P in chamber 17. Rubber diaphragm 17fprovides a closure for chamber 17 at the opposite end.

The diaphragms 17e and 17f, along with housing 17d and the particles inthe chamber, have a natural resonance frequency which is preferablymatched by the speaker 17b. It has been found that resonance providesconstant controllable aerosolization of the particles P₁ in the coatingsection 17a. The natural frequency of the chamber 17 can be varied as afunction of the length and diameter of the chamber 17 as well as theresiliency or lack thereof of the end closures, which are preferablyflexible diaphragms 17e and 17f. The weight of the particles P alsoaffect the aerosolization of the particles P₁.

The powder P₁ can be collected and reused. The casing 17g collects thepowder which escapes from chamber 17 through openings 17h. Casing 17ghas openings 17i which are covered to prevent escape of the powder P₁ tothe atmosphere.

The chamber section 17d is provided with a gas inlet 26. The casing 17gis provided with a gas outlet 21 through the filter 18. The gas ispreferably dry air or nitrogen although other gases can be used. The gasaids in aerosolizing the powder P₁ in the chamber section 17d andcoating section 17a and along with the diaphragms 17e and 17f andspeaker 17b. The powder P is aerosolized to P₁ by the speaker 17b.

The impregnated tow 10b passes through a heater 19, controlled by atemperature controller 20 and then through guides 21 and nip roller 22which controls the width of the prepreg tape 10c before being wound on atakeup drum 23.

The speakers 14a and 17b are separately controlled by a frequencygenerator 24 and power amplifier 25. It will be realized that thespeakers 14a and 17b could have separate generators and amplifiers (notshown).

FIG. 3 shows another type of coating chamber 30 for coating the tow offibers 10b. The chamber 30 is covered by square cross-sectioned casing31. The casing 31 has a gas vent 31a. The chamber 30 has an inlet 32 fora gas and a diffuser screen 33 to prevent back flow of particles intopipe 32. The chamber 30 has diaphragms 30a and 30b at either end whichare vibrated by the speaker 17b. The casing 31 is provided with slots31b and the chamber 30 is provided with slots 30c for the tow 10b. Thespeaker 17b is mounted in housing 17c as in FIG. 1.

The following examples illustrate the capability of the process:

(i) Example 1 shows that the prepreg tapes made by the method arereproducible even with manual control.

(ii) Example 2 shows that the method can tailormake prepreg tapes withany desired fiber-matrix volume fraction.

(iii) Example 3 was performed with a preliminary version of the method,wherein the coating section was separate from the aerosolizer andconnected by a pipe (not shown). The fiber tow was impregnated withparticles on a step-by-step basis.

(iv) Example 4 shows prepreg tapes which can be consolidated to makevoid-free composites. It also shows that the consolidation step has theeffect of averaging out local variations in the fiber-matrix volumefractions in the prepreg tape to produce a uniform composite part.

FIGS. 4A and 4B are photomicrographs taken at two stages in the method.FIG. 4A shows a prepreg 10c of fibers. FIG. 4B shows that void freeconsolidation of the prepreg 10c is readily achievable.

EXAMPLE 1

A set of six prepregging runs were performed with a preferred version ofthe process. The materials used were a carbon fiber tow 10 (3000fibers/tow, Hercules AS4 fibers, Hercules, Inc., Magna, Utah) and apolyamide powder P (9.2+/-4 microns, Orgasol, made by ATOCHEM Inc., GlenRock, New Jersey). The carbon fiber tow 10 was unwound from a fiberspool 11 with the help of nip rollers 13. The tow 10 was then spread bythe spreader 14 which consisted of a 10"speaker (8 Ohms, 100 Watts)mounted in a plywood housing 14d. The fiber tow 10 passed over thespeaker 14a on top of which were mounted 10 polished steel rods 14b(3/8" diameter) spaced 1"apart. The rods 14b were to hold the spread tow10a in position once they are spread by the speaker 14a. The spread tow10a passed through nip rollers 15 and entered the chamber 17. Thespeaker 17b, 8 Ohms, 100 Watts) over which a cylindrical plexiglasscolumn (8.2.increment.internal diameter, 15"in length) was mounted.Vibrating rubber diaphragms 30a and 30b were placed on the ends of thechamber 30 and above the speaker box 17c. The aerosolizer was cleanedand packed with 225 grams of powder P at the beginning of the first run.Both the aerosolizer and the spreader 14 were operated at the naturalfrequency of the respective systems The tow 10b was impregnated withparticles P in the aerosolizer and then entered a heater (12"length)where the particles sintered and coalesced on the fibers The resultingprepreg tape was then wound on takeup drum 23. The distance travelled bythe fiber tow from the fiber spool 11 to the takeup drum 23 was 65inches Table I shows the values of the different variables monitored inthe method. The volume fractions were calculated by weighing 28"lengthsof the prepreg tapes 10c and comparing the weight of an equivalentlength of unimpregnated fiber tow.

                  TABLE I                                                         ______________________________________                                        PREPREGGING EXPERIMENTS - IDENTICAL                                           AEROSOLIZER AND SPREADER CONDITIONS                                           ______________________________________                                                 Aerosolizer  Spreader                                                     Duration  Amplitude  Freq  Amplitude                                                                              Freq                                 No   (mins)    (rms V)    (Hz)  (rms V)  (Hz)                                 ______________________________________                                        1    50        5.3-5.4    12.1  11       38.5                                 2    47        5.3-5.4    12.1  11       38.5                                 3    35        5.3-5.4    12.1  11       38.5                                 4    58        5.3-5.4    12.1  11       38.5                                 5    55        5.3-5.4    12.1  11       38.5                                 6    55        5.3-5.4    12.1  11       38.5                                 ______________________________________                                            Oven      Gas Vel  Tow Speed                                                                             Vol Fraction Matrix.sup.1                      No  Temp (C.) (cc/s)   (cm/s)  Standard Deviation ( )                         ______________________________________                                        1   215-217   4.34     1.73    27.8 (4.1)                                     2   215-217   0        2.45    23.5 (5.7)                                     3   215.217   5.72     2.24    23.6 (3.1)                                     4   215-217   4.42     2.09    20.5 (3.2)                                     5   215-217   4.42     2.03    22.0 (2.0)                                     6   215-217   4.42     2.23    17.3 (3.4)                                     ______________________________________                                         .sup.1 The volume fraction is calculated using material densities from th     weight measurements.                                                     

The runs have identical oven temperature, aerosolizer and spreaderconditions Run 1 starts with a clean chamber and the walls getprogressively caked with powder to a saturation level beyond which thereis no further caking Consequenty, particle pickup is higher in run 1 ascompared with the other five runs The gas velocity of run 2 is zero andthe volume fraction matrix shows a higher standard deviation than theother five runs. This indicates that a gas flow is preferred for optimumparticle entrainment. The tow 10 velocities of all the runs wereslightly different due to manual speed control of the motors used in theprocess. Run 6 indicates that the chamber needs to be replenished withpowder P. Despite these discrepancies, the average volume fractionmatrix was 22.5 with a standard deviation of 3.5. The correspondingnumbers are 22.4 and 1.5 if the first and last runs are excluded whichis a fairer test.

EXAMPLE 2

Table II shows a set of four prepregging runs all performed at differentconditions with the same fiber-matrix combination and the same versionof the method as in Example 1. The volume fractions attained prove thata judicious combination of aerosolizer conditions and tow velocity canbe employed to make a prepreg tape with any desired volume fraction ofmatrix.

                  TABLE II                                                        ______________________________________                                        PREPREGGING EXPERIMENTS - DIFFERENT                                           CONDITIONS                                                                    ______________________________________                                                 Aerosolizer  Spreader                                                     Duration  Amplitude  Freq  Amplitude                                                                              Freq                                 No   (mins)    (rms V)    (Hz)  (rms V)  (Hz)                                 ______________________________________                                        1    60        5.3-5.4    11.6  11       38.5                                 2    61        6.6-6.7    11.4  11       38.5                                 3    30        7.4-7.5    11.4  11       38.5                                 4    40        7.4-7.5    11.8  11       38.5                                 ______________________________________                                            Oven      Gas Vel  Tow Speed                                                                             Vol Fraction Matrix                            No  Temp (C.) (cc/s)   (cm/s)  Standard Deviation ( )                         ______________________________________                                        1   215-217   4.42     1.94    11.9 (3.9)                                     2   215-217   4.42     4.11    21.3 (3.8)                                     3   215-217   4.42     1.94    42.2 (4.1)                                     4   215-217   4.42     0.90    52.9 (3.2)                                     ______________________________________                                    

EXAMPLE 3

A set of twenty-one prepregging runs were performed with an alternativeversion of the process in which the fiber tow, after spreading, passedthrough a separate impregnation chamber which was supplied withentrained particles from the aerosolizer. The aerosolizer with chamber30 (3.15"internal diameter, 6.50"speaker, 8 Ohms, 50 Watts) and thespreader 14 (6.50"speaker, 8 Ohms, 50 Watts) were powered by the samefrequency generator and amplifier (39 Hz, peak-to-peak amplitude greaterthan 24 V). The prepregging in chamber 30 was done in a semi-continuousmode i.e. the tow was stationary from time to time to give an effectiveresidence time of 2 minutes in the impregnation chamber The heater washeld at 200° C. In this version of the method, nip rollers were notused. The volume fraction of the matrix was computed to be 51.8 with astandard deviation of 3.9, excluding four runs during which there wereproblems with fiber motion.

EXAMPLE 4

The prepreg tapes were laid up manually in a two-part mold andconsolidated under pressure at 195°-200° C. for 20 minutes each. Theresulting specimens were analyzed for volume fraction of fiber under anoptical microscope using image analysis techniques. The results aresummarized in Table III. The first three samples which were consolidatedat 350 psi (2:414 Mega Pascals (MPa)) showed a few voids and a higherstandard deviation in volume fraction fiber than the last two samples at875 psi (6.034 MPa) which were essentially void-free as shown in FIG.4B. The design of optimum consolidation cycles of powder-impregnatedprepreg tapes results in the manufacture of void-free composites with avery uniform distribution of fibers across the entire composite.

                  TABLE III                                                       ______________________________________                                        CONSOLIDATION EXPERIMENTS                                                              PRESSURE    VOL FRACTION FIBER (%)                                   SAMPLE # (psi)       (STANDARD DEVIATION)                                     ______________________________________                                        1        350         65.79 (3.91)                                             2        350         69.79 (1.69)                                             3        350         70.71 (4.65)                                             4        875         74.18 (1.16)                                             5        875         73.48 (1.05)                                             ______________________________________                                    

There are cost advantages to the present method. First there is littlecapital investment. The system uses standard acoustic componentsincluding speakers 14a and 17b and generators and amplifiers 24 and 25and standard synchronous drive motors for nip rollers 13, 15 and 22.Little energy is required for operating the system. The powerrequirements are low.

The method for fixing the polymer particles to the fiber surface in theaerosolizer prior to consolidation is an important component of themethod and a variety of methods can be used. Currently the naturalcharging of the small polymer particles P₁ creates an electrostaticattraction which causes adherence to fibers 10b. Since the chargingcharacteristics of the powders P₁ can vary with their composition,environmental and atmospheric conditions, it is desirable to improvethis method or to develop an alternative method for attracting and/oradhering the particles to the fibers 10b.

Imposed electrostatics, preheating of the fibers and the use of fibercoatings as adhesives to hold the particles in position during or afterthey impact the individual fibers 10b can be used. If a largeelectrostatic charge is used to attract polymer particles P₁,fluctuations in charge carrying capacity of the powder and atmosphericconditions can be overriden. Alternatively a tubular furnace (not shown)placed at the entrance to the coating chamber 17 can be used to heat thefibers 10b prior to their entry into the coating chamber 17. Acombination of thermal input, speed of the fiber tow 10 and heat offusion of the polymer can provide conditions in which a particle P₁ willlocally melt on impact and solidify in position on the fiber 10b.Coatings on the tow 10 can also be used. Common practices from thetextile industry are used to apply 100 nanometer thick coatings of aresin onto fibers 10b. If a material which has a melting point below thepolymer particle P₁ is used as a coating, it is dry and not tacky atroom temperature to allow for handling. The fiber tow could be spreadand handled normally. Once spread, the tow 10a can be heated to raisethe temperature of the coating to above its melting point prior toentering the coating chamber 17. Impacting powder particles P₁ adhere tothis coating within the coating chamber 17 and remain in place.

The fluidization and transport of particles varies with their materialproperties and size. The acoustic fluidization of particles by size aswell as their dispersability within the fiber tow is variable by themethod of the present invention. A 10 to 50 mm prepreg 10c was producedin the Examples; however, there is no theoretical limit to the width.

The process is controllable by automated control Instrumentation formeasurement and control of acoustic energy, thermal energy, particlesize, flow rates and tow speed can be added to the process The systemcan be applied to any powder matrix used for composite materials sincematerial physical properties (density, size, acoustic power, tow speed,etc.) can be used as the input for these control algorithms.

The advantage of this system lies in the ability to make prepreg inwhich the powder particles are close together and have to flow over onlymicroscopic distances in order to consolidate Both microwave processingand thermal approaches developed can be used to heat the fibers coatedwith the particles.

A significant advantage of this system is its application to choppedfibers The acoustic fluidization can be used to suspend chopped fibersalong with the powders. The comingled fiber/powder combination can thenbe deposited onto a moving surface and can be processed to produce achopped fiber composite. Chopped fibers can also be provided on asupport screen (not shown) and the powder deposited on the fibers asthey are conveyed through the chamber 17 or 30. The chopped fiberorientation can also be controlled which results in the ability to makean aligned chopped fiber prepreg This has significant advantages formaking higher strength and stiffness composites with chopped fibers thanare obtained with random fiber composites.

Preferably the particles bonded on the fibers represent about 10% to 65%by volume of the fiber volume. Most preferably particle volume is 20 to40% of the fiber volume.

The prepreg has beads of the polymer from the particles along each fiberor group of fibers The beads are spaced uniformly on average about 1 to1000 microns apart along the fiber(s), depending upon the fiber diameterand the particle size.

It is intended that the foregoing description of the present inventionbe only illustrative and that the present invention be limited only bythe hereinafter appended claims.

We claim:
 1. In a system structured and arranged to coat particles of amaterial on a fiber the improvement which comprises:(a) chamber meansstructured and arranged around the fiber for containing the particles tobe deposited on the fiber; (b) vibrating means including opposeddiaphragm means activated by a frequency in a range selected fromaudible and ultrasonic frequencies mounted on the chamber means so thatwhen the vibrating means is actuated the particles are aerosolyzed byvibrations of the diaphragm means within the chamber means and depositedon the fiber; and (c) support means for providing the fiber in thechamber means for coating by the aerosoluzed particles.
 2. The system ofclaim 1 wherein a gas supply and exhaust means is provided in thechamber means which facilitates aerosolyzation of the particles.
 3. In asystem for coating particles of a material on a fiber, the improvementwhich comprises:(a) chamber means around the fiber for containing theparticles to be deposited on the fiber; (b) acoustic speaker means forvibrating opposed diaphragm means mounted on the chamber means so thatwhen the speaker means is activated the particles are aerosolyzed withinthe chamber means and deposited on the fiber by the vibrating diaphragmmeans; and (c) means for providing the fiber int he chamber means forcoating by the aerosolyzed particles.
 4. The system of claim 3 wherein agas supply and exhaust means is provided in the chamber means whichfacilitates aerosolyzation of the particles.
 5. In a system structuredand arranged to coat particles of a material on a continuous length offiber which is to be conveyed through the system the improvement whichcomprises:(a) chamber means structured and arranged around the fiber forcontaining the particles to be deposited on the fiber; (b) vibratingmeans including opposed diaphragm means activated by a frequency in arange selected from audible and ultrasonic frequencies mounted on thechamber means so that when the vibrating means is activated theparticles are aerosolyzed within the chamber by vibrations of thediaphragm means and deposited on the fiber as the fiber is conveyedthrough the system; and (c) conveying means for conveying the fiberthrough the chamber means.
 6. The system of claim 5 wherein a gas supplyand exhaust means is provided in the chamber means which facilitatesaerosolyzation of the particles.
 7. In a system structured and arrangedto coat particles of a material on a fiber which is to be conveyedthrough the system, the improvement which comprises:(a) chamber meansaround the fiber for containing the particles to be deposited on thefiber; (b) acoustic speaker means for vibrating opposed diaphragm meansmounted on the chamber means so that when the speaker means is actuatedthe particles are aerosolyzed within the chamber means and deposited onthe fiber by the vibrations of the diaphragm means as the fiber isconveyed through the system when the speaker means is activated; and (c)conveyer means for conveying the fiber through the chamber means.
 8. Thesystem of claim 7 wherein a gas supply and exhaust means is provided inthe chamber means which facilitates aerosolyzation of the particles. 9.A system structured and arranged to spread fibers and to coat a powderon the fibers of a fiber tow to be conveyed through the system whichcomprises:(a) feed means for feeding the tow of fibers; (b) spreadermeans for spreading the tow of fibers from the feed means whilesupporting the tow as it spreads to provide a tow of spread fibers; (c)chamber means structured and arranged around the tow of spread fibersfor containing a powder to be deposited on the tow of spread fibers; (d)an acoustic speaker means for vibrating opposed diaphragm means mountedon the chamber means so that when the speaker means is activated thepowder is aerosolyzed within the chamber means and directed at the towof spread fibers and deposited on the the tow of spread fibers by thevibrations of the diaphragm means; and (e) take-up means for the coatedtow of fibers.
 10. The system of claim 9 wherein a heater means isprovided around the coated tow of fibers to aid in bonding the powder tothe coated tow of fibers.
 11. The system of claim 10 wherein the heatermeans is between the take-up means and the chamber means.
 12. The systemof claim 9 wherein the spreader means is a speaker means mountedadjacent to the tow of spread fibers.
 13. The system of claim 9 whereinthe chamber means has an inlet and outlet for a gas which aids inaerosolyzing and dispersing the powder around the tow of spread fibers.14. The system of claim 9 wherein pinch rollers are provided adjacent aninlet and an outlet of the spreader means which maintains the spread ofthe tow of spread fibers.
 15. The system of claim 9 wherein the chambermeans has a guide means at an inlet to the chamber means for the tow ofspread fibers to maintain the spread of the tow of spread fibers fromthe spreader means.
 16. The system of claim 9 wherein the spreader meansis a speaker means mounted adjacent to the tow of spread fibers, whereinthe chamber means has an inlet and outlet for a gas which aids inaerosolyzing and dispersing the powder around the tow of spread fibers,and wherein a heater means is provided around the coated tow of fibersbetween the take-up means and the chamber means to aid in bonding thepowder to the coated tow of fibers.
 17. The system of claim 16 whereinthe chamber means has a guide at an inlet to the chamber means tomaintain the spread of the tow of spread fibers from the spreader means.